Sydney is tipped to reach 37-39 degrees while the far west could reach 42 degrees.

RFS Deputy Commissioner Rob Rogers said an army of firefighters was ready.

"We've got literally thousands of firefighters ready to jump on fires as soon as they start tomorrow," he told ABC Radio.

Two air cranes will arrive in Sydney on Thursday from Greece, but they will require checks and won't be operational until next week. The National Parks and Wildlife Service is also preparing for the heightened risk and will close a number of areas in the Blue Mountains and Wollemi national parks.

NPWS will also have an aerial response team ready to respond.

All tracks and trails in Sydney's metropolitan national parks will also be closed on Thursday.

While picnic and parking areas will remain open, all bush camping sites in the Royal National Park will be closed ahead of the extreme fire conditions.

Access to North Head at Manly, a popular vantage point for the International Fleet Review, will be closed and spectators are advised to watch the tall ships depart Sydney from Dobroyd Head and South Head.